The difficulties encountered in the development of methods and compositions which continuously release pharmaceutical preparations in a uniform manner over extended periods of time are well known (see, for example, Remington's Pharmaceutical Sciences, 15th Edition, (1975), pages 1618-1635).
Recent developments in the area of controlling the release of drugs are disclosed in European Patent Application 81305426.9 and European Patent Application 82300416.3 which describe methods for controlling the release of drugs by microencapsulation and containment within a biodegradable matrix, respectively.